It is known to produce synthetic resin (thermoplastic) sheet materials by extruding the thermoplastic material and then expanding the extruded strand by blowing it, i.e. the production of a blown-foil tube. The latter, after it has been cooled sufficiently to become dimensionally stable, must be flattened to permit further processing and/or coiling in a roll.
For this purpose an apparatus designed to flatten blown-foil tubes in connection with the manufacture of plastic foils can comprise two facing or juxtaposed steel frames that are equipped, ladder fashion, with flattening assemblies.
The steel frames facing each other may form a wedge-shaped flattening zone, or may be arranged parallel to each other. Their lengths conform to the specific requirements of each plant; their widths--at least in the upper area--conform to the width of the flattened blown-foil tube.
Currently, the industry makes use of an apparatus, made up of single flattening elements which are arranged ladder fashion, and attached to longitudinal bars of the respective frame.
However, according to the type of material used and other plant conditions, the flattening elements must be modified to suit varying needs and flattening functions.
Thus, known flattening elements for engagement with a blown-foil tube, can include straight rounded-edge wood bars, which press against the plastic foils take place.
It is also known to provide flattening elements which are constructed more or less in a curved fashion--rather than straight.
Finally, there are flattening elements constructed in the form of rolls, so that the plastic foils are subjected to a rolling type friction. Particularly in the production of plastic foils made of polyethylene such rolls are used.
On the other hand, the industry makes use of plants where the extrusion unit permits the production of different plastics. In this type of foil-blowing installation, when changing from one type of plastic to another, steel frames have to be replaced with an obvious increase in production cost, to substitute suitable foil-engaging elements.